You can access a control by it's name property through the container's control collection:
Example showing the ContainsKey function to make sure the control's name does exist in the collection:
Panel polje1 = new Panel() { Name = "polje1" };
this.Controls.Add(polje1);
if (this.Controls.ContainsKey("polje1")) {
this.Controls["polje1"].BackColor = Color.Red;
}